What should one do who suffers from extreme shyness, confusion, intense fear, and obsessive thoughts of showing off (riya') when leading people in prayer, which makes him lose his devotion (khushu') and focus?

This situation requires striving against oneself to overcome it. Ward off shyness and intrusive thoughts, and do not pay attention to them. Try to feel sincerity in your worship, and beware of abandoning good deeds for fear of showing off, for that is one of Satan's tricks to hinder you from obedience.